1) Field of the Invention
The present invention relates to an optical device used in an optical communications system, and more particularly, to an optical device effective for miniaturizing an optical circuit for monitoring an optical output.
2) Description of the Related Art
An optical waveguide device is a device which implements various functions by using an optical waveguide that confines light in and causes the light to travel through a high-refractive-index are a formed in an dielectric medium. For instance, an optical waveguide device-which forms a Mach-Zehnder interferometer through use of a dielectric such as lithium niobate (LiNbO3: hereinafter simply described as LN)—is widely used as an optical modulator, an optical switch, or a variable optical attenuator, by virtue of having a very high electro-optic constant and achieving a higher response speed as compared with a device exhibiting a thermal optic (TO) effect.
However, the optical waveguide device using a dielectric substrate, such as LN, is known to be potentially susceptible to a phenomenon called temperature drift involving a shift of an operating point due to a temperature change, or a phenomenon called DC drift involving a shift of an operating point due to flow of a DC current. If the operating point has shifted for reasons of the temperature drift or the DC drift, an optical output characteristic of the optical waveguide device will fluctuate. This makes it impossible for, e.g., an optical modulator, to perform modulation in a constant state at all times.
Specifically, an optical output of an optical modulator of Mach-Zehnder type changes in accordance with cos2(Δφ/2). The parameter Δφ is the amount of change in phase given by an interaction section of the Mach-Zehnder interferometer. When the Z-cut LN substrate is used, the parameter is expressed as Δφ={π·ne3·γ33·l/(λ·d)}·V. Here, “ne” designates a refractive index of the optical waveguide; γ33 designates an electro-optic constant; “l” designates the length of electrodes provided on two parallel optical waveguides; λ designates the wavelength of light; “d” designates a distance between the electrodes; and V designates an applied voltage. The optical output characteristic of the optical modulator assumes the form of a curve as shown in FIG. 24, where the horizontal axis represents the applied voltage V.
The operating point of such an optical modulator is preferably set, in normal times, so as to come into an intermediate state between an ON state and an OFF state when the voltage applied to the electrodes is 0. However, the actual operating point often deviates from a desired point for various causes, such as a manufacturing error or various stresses. When the operating point has caused such a deviation, the operating point is usually adjusted to a desired operating point by application of a DC bias.
However, the operating point having been adjusted by the DC bias is shifted by a DC drift such as that mentioned previously. Therefore, in order to make the operating point stable, the optical output must be monitored at all times, and the DC bias must be controlled on the basis of the monitoring result. Monitoring of such an optical output is not limited solely to the application of the optical modulator. For instance, even a variable optical attenuator of Mach-Zehnder type is required to adjust the amount of optical attenuation in accordance with a temperature change or the like.
Incidentally, in the field of such an optical waveguide device, an optical waveguide device of butt-joint type is known, wherein an end face of an optical waveguide is butt-joined directly to an output optical fiber for guiding an exiting optical signal to the output optical fiber. For instance, as shown in FIG. 25A, in the butt-joint-type optical waveguide device an output optical fiber 110 is fastened to an exit end face of an optical waveguide 101A, which is capable of outputting main signal light, in a substrate 100 having the optical waveguide 101A and an optical waveguide 101B formed therein through use of a fiber-fastening member 120 such as a V-groove fiber block or a glass ferule, thereby ensuring strength of connection of the output optical fiber to the end face of the optical waveguide.
In the optical waveguide device of butt-joint configuration as shown in FIG. 25A, another conceivable measure for monitoring an output of the optical waveguide 101B is provision of, e.g., a light-receiving element 130, on the back of the fiber fastening-member 120 (i.e., a side of the fiber fastening member opposite to the optical waveguide device). However, interference attributable to the arrangement of the output optical fiber 110 and the light-receiving element 130 makes it difficult to place the light-receiving element 130 at a position where the element can sufficiently receive monitoring light. Further, the fiber-fastening member 120 hinders the light-receiving element 130 from sufficiently receiving the monitoring light output from the monitoring-side optical waveguide 101B.
The technique described in Patent Document 1 provided below is for preventing occurrence of interference, which would otherwise be attributable to the arrangement of the output optical fiber 110 and the light-receiving element 130, both being shown in FIG. 25A. Even in this technique described in Patent Document 1, monitoring light having passed through or reflected from a reinforcement capillary corresponding to the fiber fastening member 120 shown in FIG. 25A is received. Hence, receipt of the monitoring light of sufficient level is difficult.
In order to enable receipt of the monitoring light of sufficient level, insertion of an optical fiber for monitoring light into the fiber-fastening member 120 is also conceivable as shown, e.g., FIG. 25B. However, as a result the fiber-fastening member 120 has a complicated structure, which in turn entails a hike in costs of the optical waveguide device.
In relation to such a configuration of butt-joint type, one effective measure for solving the problem of interference attributable to the arrangement of the light-receiving element and for monitoring an optical output is to guide the monitoring light from a side of the substrate differing from the side from which the main signal light of the optical waveguide device is output (a lateral surface in the configuration shown in FIG. 25 located close to or away from the viewer). More specifically, as shown in FIG. 26A or 26B, guidance of the monitoring light using a curved waveguide is conceivable.
In the LN modulator shown in FIG. 26A, the width “w” of the substrate 100 is set to 1 mm to 2 mm or thereabouts, and a curvature radius Rc of the curved waveguide 101B is set to 30 mm or more, whereby the light having traveled through the monitor-side curved waveguide 101B is guided at an angle which prevents the light from undergoing total reflection on the side surface of the substrate. Alternatively, in the LN converter shown in FIG. 26B, the width “w” of the substrate 100 is set to 1 mm to 2 mm or thereabouts, and the curvature radius Rc of the curved waveguide 101B is limited to 2 mm or less, to thus prevent occurrence of a radiation loss in the curved waveguide
[Patent Document 1] Japanese Patent Laid-open 2002-182050
However, as shown in FIG. 26A, when the monitoring light is guided from the side surface of the substrate differing from the side surface from which the main signal light is output, the side surface of the substrate can prevent occurrence of total reflection of the monitoring light. However, the monitoring light is radiated outside the waveguide at any point on the curved waveguide 101B, which sometimes hinders receipt of the monitoring light of a sufficient level. As shown in FIG. 26B, when the monitoring light is guided, occurrence of a radiation loss such as that mentioned above, which would otherwise arise in the curved waveguide 101B, can be prevented. However, the monitoring light undergoes total reflection on the side surface of the substrate, and the substrate must be increased in size.
Therefore, if the curved waveguide is formed simply for the purpose of guiding the monitoring light from the lateral surface of the substrate differing from the side surface from which the main signal light is output, as shown in FIGS. 26A and 26B, there will arise a problem of difficulty being encountered in receiving sufficient monitoring light.
Moreover, if the curved waveguide is simply formed as shown in FIGS. 26A and 26B, in some cases a packaging position must be adjusted with comparatively high accuracy for ensuring a required light receiving level of the light-receiving element, because of a narrow range of a position on the lateral surface of the substrate where the receiving level of the monitoring light by the light-receiving element becomes maximum. In this case, there will also arise a problem of an increase in manufacturing costs and a decrease in the freedom of packaging design.